Cost-efficient high power pecvd deposition for solar cells

ABSTRACT

A method for forming a photovoltaic device includes providing a substrate. A layer is deposited to form one or more layers of a photovoltaic stack on the substrate. The depositing of the amorphous layer includes performing a high power flash deposition for depositing a first portion of the layer. A low power deposition is performed for depositing a second portion of the layer.

RELATED APPLICATION DATA

This application is a divisional application of U.S. patent applicationSer. No. 13/468,292 filed on May 10, 2012, incorporated herein byreference in their entirety.

BACKGROUND

1. Technical Field

The present invention relates to photovoltaic devices and methods forfabrication, and more particularly to systems, devices and fabricationmethods that improve efficiency by employing dual power depositionsteps.

2. Description of the Related Art

A barrier height is a difference between work functions of differentmaterials. The barrier height is affected by the type of material withwhich the semiconductor is in contact. A band offset is the measure ofmisalignment between energy levels at the interface between two solids.The offset between an electrode and a semiconductor is called a“Schottky barrier”. The barrier height and offset are measures of howmuch a given material resists the flow of electrical charge through amedium. Both semiconductor-semiconductor band offset andsemiconductor-electrode Schottky barrier decrease solar cell efficiency.

To improve solar efficiency, barrier heights and barrier offsets may beaddressed by material selection. However, material selection alone isoften inadequate as an appropriate material may not exist with therequired characteristics to provide an optimal transition in energybands between adjacent materials.

SUMMARY

A method for forming a photovoltaic device includes providing asubstrate. A layer is deposited to form one or more layers of aphotovoltaic stack on the substrate. The depositing of the amorphous ormicrocrystalline layer includes performing a high power flash depositionfor depositing a first portion of the layer. A low power deposition isperformed for depositing a second portion of the layer.

Another method for forming a photovoltaic device includes providing asubstrate and depositing a buffer layer between a transparent electrodeformed on the substrate and a p-type layer of a photovoltaic stack, thedepositing of the buffer layer including the steps of: performing a highpower flash deposition for depositing a first portion of the bufferlayer to increase crystallinity and conductivity of the buffer layer;and performing a low power deposition for depositing a second portion ofthe buffer layer and having a more amorphous form.

A plasma enhanced chemical vapor deposition (PECVD) system includes avacuum chamber configured to receive a substrate for forming aphotovoltaic device. A first power generator is configured to generatean electric field in the vacuum chamber for performing a high powerPECVD flash deposition for depositing a first portion of a layer on thesubstrate. The layer being included in one or more layers of aphotovoltaic stack. A second power generator is configured to generatean electric field in the vacuum chamber for performing a low power PECVDdeposition for depositing a second portion of the layer on thesubstrate.

A photovoltaic device includes a substrate; and at least one layerincluding a buffer layer or a p-type layer of a photovoltaic stack onthe substrate. The at least one layer includes a first portion of thelayer and a second portion of the layer. The first and second portionsof the layer include different crystallinity levels, and the differentcrystallinity levels are provided adjacent to other layers such that useof the different crystallinity levels better transitions band gapenergies between layers.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a cross-sectional view of a photovoltaic device with a bufferlayer formed in accordance with the present principles;

FIG. 2 depicts a graph showing current density (mA/cm²) versus voltage(V) for three different deposition powers for forming a buffer layer todemonstrate an effect of deposition power on the buffer layer;

FIG. 3 depicts a graph showing open circuit voltage (V_(OC)) versusp-layer deposition power (W) to demonstrate an effect of depositionpower on the p-layer;

FIG. 4 depicts a graph showing band gap energy (eV) versus depositionpower (W) to demonstrate an effect of p-type layer deposition power onthe intrinsic layer;

FIG. 5 is a block diagram showing a plasma enhanced chemical vapordeposition (PECVD) system for providing a dual power deposition processin accordance with the present principles; and

FIG. 6 is a block/flow diagram showing methods for fabricating aphotovoltaic device using a dual power deposition in accordance withillustrative embodiments.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In accordance with the present principles, methods and devices arepresented that provide improved efficiency based upon depositionparameters of layers formed for semiconductor devices and in particularfor photovoltaic cells. Layers for semiconductor devices are oftendeposited using low deposition energies. Low deposition energies arefavored by manufacturing since the low energy permits high qualitya-Si:H layer deposition and less energy results in lower process costs.Designers also prefer low deposition energies since the low energiescreate plasma with less energy bombardment on a surface resulting inless damage to existing structures on semiconductor devices. In thinfilm Si solar cell applications, high deposition energies are desiredfor depositing a p+ layer since the higher energies increase thelikelihood that a microcrystalline phase will form. The microcrystallinephase can reduce barrier heights between the p+ layer and an electrode(e.g., formed from a transparent conductive oxide (TCO)).

For example, photovoltaic stacks and, in particular, p-i-n structures (ap-type layer, an intrinsic layer, an n-type layer) or n-i-p structures(an n-type layer, an intrinsic layer, a p-type layer) include layerswhere a band offset at an interface can be improved based upon thecrystallinity of the layers—the higher power, the higher thecrystallinity. Higher crystallinity permits for tunneling currentthrough the barrier. In useful examples, such improvements can be gainedat an interface between a transparent conductive oxide (TCO) to the p+layer. A band offset at the TCO/p+ layer is unavoidable since alldeveloped TCO films are n-type. In particularly useful embodiments,photovoltaic devices are constructed using materials and processes thatadjust deposition power for the formation of layers, e.g., a bufferlayer at the TCO/p+ interface. To provide improved efficiency, themanner in which the buffer layer and other layers are formed may beaddressed in accordance with the present principles. In one example, abuffer layer is deposited between the TCO and the p+ layer of the deviceusing a deposition process designed to improve device efficiency. Inparticularly useful embodiments, low power deposition may be employed todeposit a bulk portion of the layer followed by a high power flashdeposition to finish the layer.

Methods and devices provide a deposition process and power settings thatimprove device efficiency. In one embodiment, a buffer layer may includeGe or Ge_(x)Si_(1-x), which is disposed between the TCO layer and the p+layer of the solar cell. The Ge is deposited using power depositionsettings that are adjusted to reduce band offsets between materials andto increase crystallinity of the Ge or GeSi buffer layer. The Ge ispreferably deposited using a high power deposition process to affect theproperties of the buffer layer to increase efficiency of the device. Thehigh power deposition process will be implemented as a flash processlasting less than about 5 seconds. In another embodiment, the p+ layeror p-type layer is deposited or processed to reduce band offsets by aflash high deposition power process. This may be performed in additionto or instead of the buffer layer processing. Other layers in thestructures may also be processed using the high deposition power flash.

In accordance with the present principles, layers are deposited by highpower plasma deposition, and, in particular, a high power plasmaenhanced chemical vapor deposition (PECVD) process. In one embodiment,plasma is provided by a dual power system. A low power system suppliesplasma for a regular deposition while a second power supply provides ahigh power short duration flash deposition. In another embodiment, ahigh power supply system is locally installed to deposit layers, e.g.,provide a scanning deposition of p-i-n (n-i-p) layers on a photovoltaicdevice.

It is to be understood that the present invention will be described interms of a given illustrative architecture having substrates andphotovoltaic stacks; however, other architectures, structures,substrates, materials and process features and steps may be variedwithin the scope of the present invention.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

A design for a photovoltaic device may be created for integrated circuitintegration or may be combined with components on a printed circuitboard. The circuit or board may be embodied in a graphical computerprogramming language, and stored in a computer storage medium (such as adisk, tape, physical hard drive, or virtual hard drive such as in astorage access network). If the designer does not fabricate chips orphotovoltaic devices, the designer may transmit the resulting design byphysical means (e.g., by providing a copy of the storage medium storingthe design) or electronically (e.g., through the Internet), directly orindirectly.

Methods as described herein may be used in the fabrication ofphotovoltaic devices and/or integrated circuit chips with photovoltaicdevices. The resulting devices/chips can be distributed by thefabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged devices/chips), as a bare die, or in a packagedform. In the latter case the device/chip is mounted in a single chippackage (such as a plastic carrier, with leads that are affixed to amotherboard or other higher level carrier) or in a multichip package(such as a ceramic carrier that has either or both surfaceinterconnections or buried interconnections). In any case, thedevices/chips are then integrated with other chips, discrete circuitelements, and/or other signal processing devices as part of either (a)an intermediate product, such as a motherboard, or (b) an end product.The end product can be any product that includes integrated circuitchips, ranging from toys, energy collectors, solar devices and otherapplications including computer products or devices having a display, akeyboard or other input device, and a central processor.

It should be further understood that material compounds will bedescribed in terms of listed elements, e.g., SiC, SiGe, etc. Thesecompounds include different proportions of the elements within thecompound, e.g., Si_(x)C_(y) where x/y≠1, etc. In addition, otherelements may be included in the compound and still function inaccordance with the present principles.

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 1, an illustrativephotovoltaic structure 100 is illustratively depicted in accordance withone embodiment. The photovoltaic structure 100 may be employed in solarcells, light sensors or other photovoltaic applications. Structure 100includes a substrate 102 that permits a high transmittance of light. Thesubstrate 102 may include a transparent material, such as glass, apolymer, etc. or combinations thereof.

A first electrode 104 includes a transparent conductive material.Electrode 104 may include a transparent conductive oxide (TCO), such as,e.g., a fluorine-doped tin oxide (SnO₂:F, or “FTO”), doped zinc oxide(e.g., ZnO:Al), indium tin oxide (ITO) or other suitable materials. Forthe present example, a doped zinc oxide is illustratively employed forelectrode 104. The TCO 104 permits light to pass through to an activelight-absorbing material beneath and allows conduction to transportphoto-generated charge carriers away from that light-absorbing material.The TCO 104 may be deposited by a chemical vapor deposition (CVD)process, a low pressure CVD (LPCVD) process or a plasma-enhanced(PE-CVD) process.

A buffer layer 105 may be employed. Buffer layer 105 may includegermanium or silicon germanium and may include a p-type dopant such asB, Ga, In or combinations thereof. In one embodiment, the buffer layer105 includes a silicon-germanium alloy layer including germanium,silicon, optionally a p-type dopant, and hydrogen. In this embodiment,the atomic concentration of germanium may be greater than 50%. Dependingon the work-function of TCO 104, Si content in Ge can be varied. Thebuffer layer 105 has a greater atomic concentration of germanium than ap-doped layer 106 formed on the buffer layer 105.

The buffer layer 105 can be amorphous, microcrystalline, or singlecrystalline. The buffer layer 105 may include a hydrogenated material.For example, if the buffer layer 105 includes a hydrogenated amorphoussilicon germanium alloy, a hydrogenated microcrystallinesilicon-germanium alloy, a hydrogenated amorphous germanium, or ahydrogenated microcrystalline germanium, the hydrogenation of thematerial of the buffer layer 105 decreases localized electronic statesand increases the conductivity of the buffer layer 105. A hydrogenatedamorphous germanium may be formed using GeH₄ plasma.

The buffer layer 105 can be formed, for example, by plasma enhancedchemical vapor deposition (PECVD). In accordance with the presentprinciples, the buffer layer 105 is formed with a dual process includinga low power deposition and a high power flash deposition. In oneembodiment, the low power deposition may include a power of betweenabout 1.0 mW/cm² and about 100 mW/cm², and the high power flashdeposition may include a power of between about 100 mW/cm² and about 100W/cm². The deposition temperature for either process may be maintainedat between about 200 degrees C. to about 300 degrees C., although othertemperatures may be employed. Either process may be performed firstdepending on the desired crystallinity of the completed layer. The highpower deposition includes a power higher than the low power depositionto provide thicknesses of the same layer that have different levels orcrystallinity. This better transitions band gap energies between layers.

The thickness of the buffer layer 105 can be from about 1 nm to about 30nm, although lesser and greater thicknesses can also be employed. Allthe barriers, e.g., between TCO 102 and the buffer layer 105 and betweenthe buffer layer 105 and the p+ layer 106, exist at valence bandlocations. One purpose of having the buffer layer 105 is to make amid-gap bridge between TCO 102 and p+ layer 106.

In the case of FIG. 1, the low power deposition is performed firstfollowed by the high power flash to increase crystallinity towards thep+ layer 106. In this case, the buffer layer 105 includes an interfaciallayer 116 with higher crystallinity (microcrystalline) than theremaining portions of the buffer layer 105. The buffer layer 105, withthe interface layer 116, better aligns the Fermi level between the TCOlayer 104 (e.g., ZnO) and the p-type layer 106 (e.g., a-SiC:H). TheFermi level of the TCO layer 104 in the case of ZnO is about 4.5 eV.Without the buffer layer 105, a larger Schottky barrier exists betweenthe TCO 104 and the p-type layer 106. With the buffer layer 105, theFermi levels of TCO 104 and the buffer layer 105 are well-alignedreducing the barrier offset. A conduction band between the buffer layer105 and the p-type layer 106 is also well-aligned. In this way, theSchottky barrier is reduced or eliminated resulting in better conductionand more efficient device operation by permitting higher open circuitvoltage (V_(OC)) without sacrificing fill factor (FF). In particularlyuseful embodiments, it is believed that increased crystallinity of thebuffer layer 105 and in particular the interface layer 116 correlateswith increased open circuit voltage (V_(OC)) while at least maintainingfill factor (FF).

The p-doped layer 106 may include an amorphous, microcrystalline, orsingle-crystalline p-doped silicon-containing material. The p-dopedlayer 106 can be a p-doped silicon layer including: silicon and at leastone p-type dopant and optionally hydrogen; a p-doped silicon-carbonalloy layer including: silicon, carbon, at least one p-type dopant andoptionally hydrogen. Other materials may also be employed.

In some cases, the p-doped layer 106 may include a hydrogenatedamorphous, microcrystalline, or single-crystalline p-dopedsilicon-containing material. A hydrogenated p-doped silicon-containingmaterial can be deposited in a process chamber having asilicon-containing reactant gas as a carrier gas. To facilitateincorporation of hydrogen in the hydrogenated p-doped silicon-containingmaterial, a carrier gas including hydrogen can be employed. Hydrogenatoms in the hydrogen gas within the carrier gas are incorporated intothe deposited material to form an amorphous or microcrystallinehydrogenated p-doped silicon-containing material of the p-doped layer106.

In one embodiment, p-doped or p-type layer 106 includes boron as adopant, although other p-type dopants may be employed. Layer 106 may bedeposited using diborane gas, trimethylborane or other gases. The p-typelayer 106 may have a thickness of between about 5-30 nm. In particularlyuseful embodiments, layer 106 includes amorphous SiC or Si. Theprocessing of the p-type layer 106 also can affect the efficiency of thedevice. For example, the deposition parameters for depositing the p-typelayer 106 may be selected to increase the effects of the buffer layer105.

The p-type layer 106 can be formed, for example, by plasma enhancedchemical vapor deposition (PECVD). As with the buffer layer 105 or anyother layer in the device 100, the two-part deposition process may beemployed that includes a low power deposition and a high power flashdeposition. As before, the low power deposition may include a power ofbetween about 1.0 and about 100 mW/cm², and the high power flashdeposition may include a power of between about 100 mW/cm² and about 100W/cm². The deposition temperature for either process may be maintainedat between about 200 degrees C. to about 300 degrees C., although othertemperatures may be employed. Either process may be performed firstdepending on the desired crystallinity of the completed layer.

The thickness of the p-type layer 106 can be from about 5 nm to about 30nm, although lesser and greater thicknesses can also be employed. Forthe smaller thicknesses, the high power flash deposition may be employedalone. For example, layer thicknesses of about 5 nm may be formed by thehigh power flash deposition.

In one embodiment, the high power flash is performed first followed bythe low power deposition. In this case, the p-type layer 106 includes aninterfacial layer 118 with higher crystallinity than the remainingportions of the p-type layer 106. The p-type layer 106 (e.g., a-SiC:H)is better aligned in terms of the Fermi level with the TCO layer 104(e.g., ZnO) or the buffer layer 105 (e.g., Ge) (if employed). Moreefficient device operation is provided by permitting higher open circuitvoltage (V_(OC)) without sacrificing fill factor (FF). In particularlyuseful embodiments, it is believed that increased crystallinity of thep-type layer 106 results in enhanced doping efficiency and in particularthe interface layer 118 correlates with increased V_(OC) while at leastmaintaining FF.

Processing continues with the formation of an intrinsic layer 110 formedon layer 106 from compatible materials. Intrinsic layer 110 may beundoped and may include an amorphous silicon material, e.g.,hydrogenated amorphous Si (a-Si:H). The intrinsic layer 110 may includea thickness of between about 100-300 nm, although other thicknesses arecontemplated. The intrinsic layer 110 may be formed using a chemicalvapor deposition (CVD) process, or a plasma-enhanced (PE-CVD), e.g.,from silane gas and hydrogen gas.

A doped layer 112 (e.g., an n-type layer) is formed on the intrinsiclayer 110. Layer 112 may include an n-type Si containing layer, e.g.,hydrogenated microcrystalline (μc-Si:H), single crystalline (Si) or anamorphous (a-Si) layer. Layer 112 may be deposited by a chemical vapordeposition (CVD) process, or a plasma-enhanced (PE-CVD). The n-typelayer 112 may have a thickness of between about 5-20 nm. A backreflector and/or bottom electrode 114 may be formed using a reflectivemetal, such as Ag, Al, etc. and may be combined with ZnO backreflectors. A suitable metal deposition process may be employed, e.g.,physical or chemical vapor deposition, sputtering, evaporation, electroor electroless plating, etc. It should be understood that other materialselections, layers, structures, etc. may be employed in device 100including additional back reflectors, etc. or instead of backreflectors: tandem cells, etc.

It should be understood that the structures depicted in FIG. 1 areillustrative and that other structures may be employed, e.g., the p-i-nstack can be reversed (n-i-p). In addition, one or more layers of thedevice 100 may be formed with the two-part deposition process. Inaddition, the high power deposition process may be employed alone toform small thickness layers.

In accordance with another embodiment, the high power flash depositionmay be employed in multiple pulses to form layers. For example, if thehigh power flash deposition provides a 3 nm thickness for the bufferlayer 105, the high power flash deposition may be repeated four times(four pulses) to achieve a 12 nm buffer layer 105. The high power flashdeposited buffer layer 105 would include a higher crystallinity, whichresults in improved efficiency of operation. The high power flashdeposited buffer layer 105 would be more cost effective since the flashpulses would take only a few seconds, and low temperature is maintainedtherefore maintaining thermal budget and reducing damage to existinglayers/structures.

Referring to FIG. 2, a graph shows general effects of differentdeposition powers for a buffer layer (e.g., buffer layer 105) in a p-i-nstack photovoltaic device. The graph shows three plots corresponding todifferent deposition powers for a buffer layer formed from a-GeSi:H. Thegraph plots current density (J) in mA/cm² versus voltage (V) in voltsfor three devices. The deposition powers include the following: 0.15W/cm² for plot 202, 0.6 W/cm² for plot 204 and 1.6 W/cm² for plot 206. Asystematic degradation of FF occurs as the deposition power is reduced.For a power of 0.15 W/cm² (plot 202), FF was 53%. For a power of 0.6W/cm² (plot 204), FF was 66%. For a power of 1.6 W/cm² (plot 206), FFwas 69%. Advantageously, V_(OC) remains high with higher FF.

The plots 202, 204 and 206 correspond to devices that were processed inthe same way but for the deposition power of the buffer layer. Thedevices included p-i-n (p-type, intrinsic, n-type) layers deposited at250 degrees C. and the p type layer (106) was deposited with a power of0.6 W/cm² for these devices. In accordance with these findings, a highpower deposition seems to result in improved device efficiency. This maybe as a result of increased crystallinity and therefore conductivity ofthe deposited material and/or other factors. However, high power isexpensive, costs thermal budget and may have a destructive effect onadjacent structures. In addition for large area deposition inmanufacturing environments, high deposition power is not favored. Largearea refers to large scale devices having an illustrative size on theorder of feet or meters, e.g., 1.5 meters by 0.6 meters. Such devicesmay include solar panels. Low power is favored in such environments.

In accordance with the present principles, a high power flash depositionwith a low power bulk deposition provides improved device efficiencywhile permitting a reduction in overall deposition power. In someembodiments, the high power flash deposition may be employed alone toform a layer depending on a thickness needed and a deposition rateprovided by the process. In other embodiments, the high power flashdeposition may be repeatedly performed in a pulsing fashion toaccumulate the needed thickness. The pulses may include a same pulserate or the pulse rate may be altered for subsequent pulses. The highpower flash deposition is preferably a plasma enhanced CVD process andmay include a duration (pulse duration) of between 1 millisec to about 5seconds. Other times are also contemplated and may depend on theapplication, materials, etc.

Referring to FIG. 3, experimental results for devices fabricated usingdifferent deposition powers are depicted to show a correlation betweendeposition power and open circuit voltage (V_(OC)) (in millivolts).V_(OC) is graphed versus deposition power (W) for a p-type layer inp-i-n photovoltaic devices. The p-type layer included a p-doped a-Silayer. In accordance with the graph, V_(OC) increased with depositionpower.

It should be understood that the deposition power may be increased tolevels higher than those depicted herein (e.g., deposition powers of 900W or greater may be provided) and that comparable improvements may beobtained but may have some limitations. In addition, other parametersmay be adjusted as well including duration of a treatment, temperature,etc. It should be understood that the high power deposition may improveother adjacent layers as well including but not limited to the intrinsiclayer, which will be illustratively described with respect to FIG. 4,and even the n-type layer.

Referring to FIG. 4, experimental results for devices fabricated usingdifferent deposition powers are depicted to show a correlation betweendeposition power of the p-type layer and band gap energy (eV) of theintrinsic layer (e.g., 110, FIG. 1). Band gap is graphed versusdeposition power (W) for a-Si p-i-n photovoltaic devices. In accordancewith the graph, band gap increased with deposition power until itreached 2.00-2-01 eV.

From FIGS. 2, 3 and 4, higher power deposition provides improvements inefficiency. However, this needs to be balanced against some drawbacksincluding lifetime efficiency, cost, ease of manufacturing, etc. Inaccordance with the present principles, a high power flash process isperformed in a PECVD chamber to form one or more of a buffer layer and ap-type layer. In particularly useful embodiments, a largest efficiencygain comes from high power flash processing of the p+ type layer and theuse of high power flash processing of a buffer layer. In oneparticularly useful embodiment, a high power flash process includes apower greater than about 100 mW/cm² for less than about 5 seconds, andmore preferably less than one second. The temperature may be about200-300 degrees C.

Referring to FIG. 5, a dual power system 300 is shown in accordance withone illustrative embodiment. The system 300 includes a low powergenerator 302, which is employed most of time, and a high powergenerator 304, which is used for a few seconds or less to provide a highpower flash process, as described. The generators 302 and 304 includepower supplies capable of generating sufficient potential between anelectrode 320 and a ground 322. The system 300 may include a vacuumchamber 324 for carrying out a PECVD processes. As is known, plasmaincludes charged particles/ions accelerated in an electric field (beingset by the power settings) to bombard and therefore be deposited on asurface of a device, wafer or panel 310. The system 300 may include asupport platform or carrier 306 for supporting the device, wafer orpanel 310 on which deposition and other processes are carried out.Platform 306 may include a conveyor or other mechanical device for amanufacturing line. This may be particularly useful for scanningdeposition processes, e.g., for fabricating solar panels. The platform306 also includes the ground 322 to provide a ground potential to theelectric field accelerating the plasma particles. Two differentelectrical feedthroughs 326 and 328, respectively, connect the highpower generator 304 and the low power generator 302 to the electrode320.

In one embodiment, the high power generator 304 is retrofit into anexisting chamber to provide the dual power system 300. In otherembodiments, a same generator may be employed to perform both the highand low power deposition processes. In still other embodiments, only thehigh power generator 304 is provided.

The high power generator 304 is configured to provide a higherconductivity to a layer by providing increased crystallinity to anotherwise amorphous or microcrystalline deposition. In one embodiment,the flash deposition may form a thickness of less than about 5 nm inless than about 5 seconds. Other thicknesses and times are alsocontemplated. The high power generator 304 provides a plasma at a powerof between about 100 mW/cm² and about 100 W/cm². Higher powers are alsocontemplated.

In other embodiments, the power generator 304 is configured to pulse toprovide a plurality of the high power flash deposition cycles to formmultiple layers of high power flash deposited material. For example, 5cycles may produce a 10 nm layer where each cycle provides 2 nm. Inother embodiments, the high power flash deposition cycles may includedifferent pulse durations. For example, a 10 nm layer may be depositedin 3 cycles: a 5 nm deposition cycle, a 3 nm deposition cycle and a 1 nmdeposition cycle. Other combinations of thickness and flash durationsare also contemplated.

Referring to FIG. 6, methods for forming a photovoltaic device areillustratively shown. It should also be noted that, in some alternativeimplementations, the functions noted in the blocks may occur out of theorder noted in FIG. 6. For example, two blocks shown in succession may,in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagram and/or flowchart illustration, and combinations of blocksin the block diagram and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts, or combinations of special purpose hardware andcomputer instructions.

In block 402, a substrate is provided. A transparent electrode may beformed on a transparent substrate. The transparent electrode may includea transparent conductive oxide or the like. The transparent substratemay include glass, a polymer, monocrystalline silicon, etc. In block404, one or more layers of a photovoltaic stack are deposited on thesubstrate. The depositing step of the one or more layers includesperforming a high power flash deposition for depositing a first portionof the layer in block 410. It should be understood that the one or morelayers may include deposited amorphous or microcrystalline layers withincreased crystallinity and conductivity as a result of the high powerflash deposition.

In block 420, a low power deposition is performed for depositing asecond portion of the layer. The low power deposition may be eliminatedif the high power deposition provides sufficient thickness for thelayer. The one or more layers processed using the dual power process mayinclude a p-type layer and a buffer layer, although the process may beemployed for other layers as well. In a particularly useful embodiment,the buffer layer and the p-type layer are formed by the dual powerprocess.

In block 412, the high power flash deposition may include depositing athickness of less than about 5 nm in less than about 5 seconds. Otherparameters are also contemplated in accordance with the presentprinciples. In block 416, the high power flash deposition may include aplasma enhanced chemical vapor deposition at a power of between about100 mW/cm² and about 100 W/cm². In block 418, the high power flashdeposition may include pulsing the high power flash deposition processto form multiple thicknesses of high power flash deposited material in asame layer. Pulses may include different pulse durations. The depositedthickness may increase in crystallinity with depth. This may beperformed by increasing the power with each subsequent pulse orgradually increasing the power over time.

The high power flash deposition may be performed before or after the lowpower deposition. The p-type layer, the intrinsic layer and the n-typelayer may be formed at a temperature of about 250 degrees Celsius. Othertemperatures may also be employed.

In block 422, processing continues to complete the photovoltaic device.For example, a back reflector or back reflectors may be formed. Otherstructures and layers are also envisioned. In one embodiment, the p-typelayer includes a form of silicon (e.g., a-Si, a-Si:H, a-SiC, a-SiC:H,crystalline forms of Si or SiC, etc.). The intrinsic layer may includea-Si, a-Si:H, or crystalline forms of Si, etc.). The n-type layer mayinclude an n-doped form of a-Si, a-Si:H, or crystalline forms of Si,etc.).

Having described preferred embodiments for cost-efficient high powerPECVD deposition for solar cells (which are intended to be illustrativeand not limiting), it is noted that modifications and variations can bemade by persons skilled in the art in light of the above teachings. Itis therefore to be understood that changes may be made in the particularembodiments disclosed which are within the scope of the invention asoutlined by the appended claims. Having thus described aspects of theinvention, with the details and particularity required by the patentlaws, what is claimed and desired protected by Letters Patent is setforth in the appended claims.

What is claimed is:
 1. A plasma enhanced chemical vapor deposition(PECVD) system, comprising: a vacuum chamber configured to receive asubstrate for forming a photovoltaic device; a first power generatorconfigured to generate an electric field in the vacuum chamber forperforming a high power PECVD flash deposition for depositing a firstportion of a layer on the substrate, the layer being included in one ormore layers of a photovoltaic stack; and a second power generatorconfigured to generate an electric field in the vacuum chamber forperforming a low power PECVD deposition for depositing a second portionof the layer on the substrate.
 2. The system as recited in claim 1,wherein the high power flash deposition forms a thickness of less thanabout 5 nm in less than about 5 seconds.
 3. The system as recited inclaim 1, wherein the first power generator provides a plasma at a powerof between about 100 mW/cm² and about 100 W/cm².
 4. The system asrecited in claim 1, wherein first power generator is configured to pulseto provide a plurality of the high power flash deposition cycles to formmultiple layers of high power flash deposited material.
 5. The system asrecited in claim 4, wherein the high power flash deposition cyclesinclude different pulse durations.
 6. The system as recited in claim 1,wherein the vacuum chamber includes a support structure for holding orconveying the substrate.
 7. A photovoltaic device, comprising: asubstrate; and at least one layer including a buffer layer or a p-typelayer of a photovoltaic stack on the substrate, the at least one layerincluding a first portion of the layer and a second portion of thelayer, the first and second portions of the layer including differentcrystallinity levels, the different crystallinity levels being providedadjacent to other layers such that use of the different crystallinitylevels better transitions band gap energies between layers.
 8. Thephotovoltaic device of claim 7, wherein the buffer layer is positionedbetween a TCO layer and the p-type layer.
 9. The photovoltaic device ofclaim 8, wherein the buffer layer is deposited using a low power andhigh power two stage deposition process.
 10. The photovoltaic device ofclaim 9, wherein the buffer layer includes a first portion in contactwith the p-type layer formed using a low power deposition to provide alow crystallinity, and the buffer layer includes a second portion incontact with the TCO layer formed using a high power deposition toprovide a high crystallinity.